Staybridge Suites Lake Buena Vista is our absolute favorite. Very clean and spacious, with full kitchens (we always get the 2-bed, 2 bath suite, but they have a 1-bed, 1 bath suite too). Way cheaper than almost any Disney property, except the value resorts and the rooms there are tiny.
Staybridge Suites LBV is also right next door to Winn Dixie, which saves you alot of time and money if you can eat some meals in the room. They also have "free" breakfast every morning. You are maybe 5 minutes from the entrance to Downtown Disney.
They have a bus schedule too, though we have never used it (we drive to Orlando).
The only caveat: The pool area is somewhat small. So . . . if your kids are really wanting to swim constantly that might not be the choice.
Also - DO NOT confuse it with the Staybridge on I-drive - NOT the same property.
Here is a link that might help you find it: http://www.ichotelsgroup.com/h/d/sb/1/en/hotel/mcobv .
Good luck and hope you have a GREAT time.
PS: We, too, have stayed at Marriott World and it's very nice. My only "beef" there is that it is a HUGE property and your room might be a long walk from the bus area. JMO.